When hydrogen and oxygen are mixed
together in a chamber at room temperature, nothing happens. There is no chemical reaction,
but if a match is lit in the chamber, the chemical reaction happens so fast that it is
explosive.
The reaction of hydrogen and oxygen to form water is spontaneous, but
it will not happen unless some energy is put into the system. The energy required is
called the activation energy. The chemical equation
2H2 + O2 –> 2H2O can be written as
2H2 + O2 –> very high energy intermediate state –>
2H2O
to indicate that for the reaction to happen the chemicals involved must transition through
a short lived high energy intermediate state.
Because this state has more energy than the initial and final states,
energy must be put into the chemicals to allow them to reach the intermediate state. In
this case, the match allows some hydrogen and oxygen atoms to transition to the
intermediate state. They then form water and release heat. The heat release causes more
hydrogen and oxygen atoms to transition to the intermediate state. The reaction builds on
itself in this manner until almost all of the hydrogen and oxygen are used up and only
water remains.
Activation energy is very important for life. It allows chemicals to
exist in a state indefinitely even if a change in state may increase the entropy of the
universe. Figure 6.13 conveys these key concepts. The reaction to create water will
increase the entropy of the universe as indicated by the arrow on the left side of the
figure. Therefore, the reaction is said to be spontaneous. Nevertheless, it is not
spontaneous unless the hydrogen and oxygen are provided with enough energy to cross
through the high energy intermediate stage.
It is the activation energy that allows life to exist. Because of this
barrier, chemicals that are not thermodynamically favored can exist for many hundreds of
years. If a process increases the entropy of the universe, then the second law defines the
process as spontaneous, but it does not have to happen right away. The process may take
years to complete. The speed of a chemical reaction depends on the activation energy. The
second law does not determine how fast a chemical reaction will happen. Hydrogen and
oxygen can coexist in a chamber for a thousand years if no energy source is present to
start the reaction.
Notice that the direction of increasing entropy is drawn downward in
figure 6.13 to indicate that water is the preferred state as it maximizes the entropy of
the universe.
Figure 6.13: Activation Energy
next: Chemical Equilibria
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